A selective inhibitor of Na+/Ca2+ exchanger, SEA0400, preserves cardiac function and high-energy phosphates against ischemia/reperfusion injury

Nongenetic Optical Modulation of Pluripotent Stem Cells Derived Cardiomyocytes Function in the Red Spectral Range

Optical stimulation in the red/near infrared range recently gained increasing interest, as a not-invasive tool to control cardiac cell activity and repair in disease conditions. Translation of this approach to therapy is hampered by scarce efficacy and selectivity. The use of smart biocompatible materials, capable to act as local, NIR-sensitive interfaces with cardiac cells, may represent a valuable solution, capable to overcome these limitations. In this work, a far red-responsive conjugated polymer, namely poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]] (PCPDTBT) is proposed for the realization of photoactive interfaces with cardiomyocytes derived from pluripotent stem cells (hPSC-CMs). Optical excitation of the polymer turns into effective ionic and electrical modulation of hPSC-CMs, in particular by fastening Ca2+ dynamics, inducing action potential shortening, accelerating the spontaneous beating frequency. The involvement in the phototransduction pathway of Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) and Na+ /Ca2+ exchanger (NCX) is proven by pharmacological assays and is correlated with physical/chemical processes occurring at the polymer surface upon photoexcitation. Very interestingly, an antiarrhythmogenic effect, unequivocally triggered by polymer photoexcitation, is also observed. Overall, red-light excitation of conjugated polymers may represent an unprecedented opportunity for fine control of hPSC-CMs functionality and can be considered as a perspective, noninvasive approach to treat arrhythmias.

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

Regulation of NCX1 by palmitoylation

Palmitoylation (S-acylation) is the reversible conjugation of a fatty acid (usually C16 palmitate) to intracellular cysteine residues of proteins via a thioester linkage. Palmitoylation anchors intracellular regions of proteins to membranes because the palmitoylated cysteine is recruited to the lipid bilayer. NCX1 is palmitoylated at a single cysteine in its large regulatory intracellular loop. The presence of an amphipathic α-helix immediately adjacent to the NCX1 palmitoylation site is required for NCX1 palmitoylation. The NCX1 palmitoylation site is conserved through most metazoan phlya. Although palmitoylation does not regulate the normal forward or reverse ion transport modes of NCX1, NCX1 palmitoylation is required for its inactivation: sodium-dependent inactivation and inactivation by PIP2 depletion are significantly impaired for unpalmitoylatable NCX1. Here we review the role of palmitoylation in regulating NCX1 activity, and highlight future questions that must be addressed to fully understand the importance of this regulatory mechanism for sodium and calcium transport in cardiac muscle.

Effects of ticagrelor on the sodium/calcium exchanger 1 (NCX1) in cardiac derived H9c2 cells

Ticagrelor is a direct acting and reversibly binding P2Y₁₂ antagonist approved for the prevention of thromboembolic events. Clinical effects of ticagrelor cannot be simply accounted for by pure platelet inhibition, and off-target mechanisms can potentially play a role. In particular, recent evidence suggests that ticagrelor may also influence heart function and improve the evolution of myocardial ischemic injury by more direct effects on myocytes. The cardiac sodium/calcium exchanger 1 (NCX1) is a critical player in the generation and control of calcium (Ca²⁺) signals, which orchestrate multiple myocyte activities in health and disease. Altered expression and/or activity of NCX1 can have profound consequences for the function and fate of myocytes. Whether ticagrelor affects cardiac NCX1 has not been investigated yet. To explore this hypothesis, we analyzed the expression, localization and activity of NCX1 in the heart derived H9c2-NCX1 cells following ticagrelor exposure. We found that ticagrelor concentration- and time-dependently reduced the activity of the cardiac NCX1 in H9c2 cells. In particular, the inhibitory effect of ticagrelor on the Ca²⁺-influx mode of NCX1 was evident within 1 h and further developed after 24 h, when NCX1 activity was suppressed by about 55% in cells treated with 1 μM ticagrelor. Ticagrelor-induced inhibition of exchanger activity was reached at clinically relevant concentrations, without affecting the expression levels and subcellular distribution of NCX1. Collectively, these findings suggest that cardiac NCX1 is a new downstream target of ticagrelor, which may contribute to the therapeutic profile of ticagrelor in clinical practice.

Transplantation of Hearts Donated after Circulatory Death

Cardiac transplantation has become limited by a critical shortage of suitable organs from brain-dead donors. Reports describing the successful clinical transplantation of hearts donated after circulatory death (DCD) have recently emerged. Hearts from DCD donors suffer significant ischemic injury prior to organ procurement; therefore, the traditional approach to the transplantation of hearts from brain-dead donors is not applicable to the DCD context. Advances in our understanding of ischemic post-conditioning have facilitated the development of DCD heart resuscitation strategies that can be used to minimize ischemia-reperfusion injury at the time of organ procurement. The availability of a clinically approved ex situ heart perfusion device now allows DCD heart preservation in a normothermic beating state and minimizes exposure to incremental cold ischemia. This technology also facilitates assessments of organ viability to be undertaken prior to transplantation, thereby minimizing the risk of primary graft dysfunction. The application of a tailored approach to DCD heart transplantation that focuses on organ resuscitation at the time of procurement, ex situ preservation, and pre-transplant assessments of organ viability has facilitated the successful clinical application of DCD heart transplantation. The transplantation of hearts from DCD donors is now a clinical reality. Investigating ways to optimize the resuscitation, preservation, evaluation, and long-term outcomes is vital to ensure a broader application of DCD heart transplantation in the future.

MicroRNA-135a regulates sodium-calcium exchanger gene expression and cardiac electrical activity

BACKGROUND

Complete atrioventricular block (CAVB) causes arrhythmogenic remodeling and increases the risk of torsades de pointes arrhythmias. MicroRNAs (miRNAs) are key regulators of gene expression that contribute to cardiac remodeling.

OBJECTIVE

The purpose of this study was to assess miRNA changes after CAVB and identify novel candidates potentially involved in arrhythmogenic cardiac remodeling.

METHODS

CAVB was induced in mice via His-bundle ablation. Expression of miRNAs was evaluated by pan-miRNA microarray with quantitative polymerase chain reaction (qPCR) confirmation, on samples obtained 24 hours and 4 weeks post-CAVB. MiRNA target prediction algorithms were used to identify potential target genes. Targets confirmed by luciferase assays in HEK293 cells were followed up with overexpression studies in neonatal rat ventricular myocytes to evaluate regulation using real time- quantitative polymerase chain reaction (RT-qPCR), western blots, cell shortening measurements, and fura-2 Ca²⁺ fluorescence imaging.

RESULTS

Of >400 miRNAs assayed, only miRNA-135a (miR-135a) was altered at 24 hours, down-regulated 78% (P <.001). Algorithms predicted miR-135a regulation of the sodium-calcium exchanger type 1 (NCX1). miR-135a transfection suppressed NCX1 3'UTR reporter activity by 42% (P <.001), mRNA expression by 34% (P <.001), and protein levels by 45% (P <.001) vs noncoding miRNA control. miR-135a overexpression reduced spontaneous beating frequency of neonatal rat ventricular myocytes by 63% (P <.001) while slowing decay (by 56%, P <.05) of caffeine-induced Ca²⁺ transients. miR-135a also suppressed the Ca²⁺ loading effects of ouabain and ouabain-induced spontaneous Ca²⁺ release events.

CONCLUSION

NCX1 is negatively regulated by miR-135a, a microRNA that is down-regulated in the heart after CAVB in mice. By controlling NCX1 expression, miR-135a modulates cardiomyocyte automaticity, Ca²⁺ extrusion, and arrhythmogenic Ca²⁺ loading/spontaneous Ca²⁺ release events. Therefore, miR-135a may contribute to proarrhythmic remodeling after CAVB.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.

EXPERIMENTAL APPROACH

Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.

KEY RESULTS

Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.

CONCLUSIONS AND IMPLICATIONS

Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling

The electrogenic Na/Ca exchanger (NCX) mediates bidirectional Ca movements that are highly sensitive to changes of Na gradients in many cells. NCX1 is implicated in the pathogenesis of heart failure and a number of cardiac arrhythmias. We measured NCX1 palmitoylation using resin-assisted capture, the subcellular location of yellow fluorescent protein–NCX1 fusion proteins, and NCX1 currents using whole-cell voltage clamping. Rat NCX1 is substantially palmitoylated in all tissues examined. Cysteine 739 in the NCX1 large intracellular loop is necessary and sufficient for NCX1 palmitoylation. Palmitoylation of NCX1 occurs in the Golgi and anchors the NCX1 large regulatory intracellular loop to membranes. Surprisingly, palmitoylation does not influence trafficking or localization of NCX1 to surface membranes, nor does it strongly affect the normal forward or reverse transport modes of NCX1. However, exchangers that cannot be palmitoylated do not inactivate normally (leading to substantial activity in conditions when wild-type exchangers are inactive) and do not promote cargo-dependent endocytosis that internalizes 50% of the cell surface following strong G-protein activation or large Ca transients. The palmitoylated cysteine in NCX1 is found in all vertebrate and some invertebrate NCX homologs. Thus, NCX palmitoylation ubiquitously modulates Ca homeostasis and membrane domain function in cells that express NCX proteins.—Reilly, L., Howie, J., Wypijewski, K., Ashford, M. L. J., Hilgemann, D. W., Fuller, W. Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling.

Na(+)-Ca(2+) exchanger targeting miR-132 prevents apoptosis of cardiomyocytes under hypoxic condition by suppressing Ca(2+) overload

During ischemia-reperfusion (IR) injury of the heart, Ca(2+) overload occurs, leading to cardiomyocyte dysfunction and eventual cell death by apoptosis. Since preventing Ca(2+) overload during IR injury has been reported to protect cardiomyocytes, interrupting Ca(2+) signaling cascades leading to Ca(2+) overload may exert protective effect on cardiomyocytes under hypoxic condition. One of the key regulators of the intracellular Ca(2+) level during IR injury is Na(+)-Ca(2+) exchanger 1 (NCX1), whose down-regulation during IR injury conferred protection of heart. In the present study, we examined whether down-regulation of NCX1 using exogenous microRNA ameliorates apoptosis of cardiomyocytes under hypoxic condition. Here, we identified miR-132 as a novel microRNA targeting the NCX1, whose expression increased during hypoxia. Delivery of miR-132 suppressed the increase of intracellular Ca(2+) in cardiomyocytes under hypoxia, and the expressions of apoptotic molecules, such as Bax, cytochrome C, and caspase 3, and the number of apoptotic cells were also decreased by exogenous miR-132 treatment. These results suggest the potential of miR-132 as an effective therapeutic agent against IR damage to heart by preventing Ca(2+) overload during hypoxic condition and warrant further studies to validate its anti-apoptotic effect in vivo.

The Na+/Ca2+ exchange inhibitor SEA0400 limits intracellular Ca2+ accumulation and improves recovery of ventricular function when added to cardioplegia

Background

The Na⁺/Ca²⁺ exchange inhibitor SEA0400 prevents myocardial injury in models of global ischemia and reperfusion. We therefore evaluated its potential as a cardioplegia additive.

Methods

Isolated rat cardiomyocytes were exposed to hypoxia (45 min) followed by reperfusion. During hypoxia, cells were protected using cardioplegia with (n = 25) or without (n = 24) SEA0400 (1 μM), or were not protected with cardioplegia (hypoxic control, n = 8). Intracellular Ca²⁺ levels were measured using Ca²⁺ sensitive dye (fura-2 AM). Isolated rat hearts were arrested using cardioplegia with (n = 7) or without (n = 6) SEA0400 (1 μM) then reperfused after 45 min of ischemia. Left ventricular (LV) function, troponin release, and mitochondrial morphology were evaluated.

Results

Cardiomyocytes exposed to hypoxia without cardioplegia had poor survival (13%). Survival was significantly improved when cells were protected with cardioplegia containing SEA0400 (68%, p = 0.009); cardioplegia without SEA0400 was associated with intermediate survival (42%). Cardiomyocytes exposed to hypoxia alone had a rapid increase in intracellular Ca²⁺ (305 ± 123 nM after 20 minutes of ischemia). Increases in intracellular Ca²⁺ were reduced in cells arrested with cardioplegia without SEA0400; however cardioplegia containing SEA0400 was associated with the lowest intracellular Ca²⁺ levels (110 ± 17 vs. 156 ± 42 nM after 45 minutes of ischemia, p = 0.004). Hearts arrested with cardioplegia containing SEA0400 had better recovery of LV work compared to cardioplegia without SEA0400 (23140 ± 2264 vs. 7750 ± 929 mmHg.μl, p = 0.0001). Troponin release during reperfusion was lower (0.6 ± 0.2 vs. 2.4 ± 0.5 ng/mL, p = 0.0026), and there were more intact (41 ± 3 vs. 22 ± 5%, p < 0.005), and fewer disrupted mitochondria (24 ± 2 vs. 33 ± 3%, p < 0.05) in the SEA0400 group.

Conclusions

SEA0400 added to cardioplegia limits accumulation of intracellular Ca²⁺ during ischemic arrest in isolated cardiomyocytes and prevents myocardial injury and improves recovery of LV function in isolated hearts.

Regulation of the Na+/Ca2+ exchanger by pyridine nucleotide redox potential in ventricular myocytes

The cardiac Na(+)/Ca(2+) exchanger (NCX) is the major Ca(2+) efflux pathway on the sarcolemma, counterbalancing Ca(2+) influx via L-type Ca(2+) current during excitation-contraction coupling. Altered NCX activity modulates the sarcoplastic reticulum Ca(2+) load and can contribute to abnormal Ca(2+) handling and arrhythmias. NADH/NAD(+) is the main redox couple controlling mitochondrial energy production, glycolysis, and other redox reactions. Here, we tested whether cytosolic NADH/NAD(+) redox potential regulates NCX activity in adult cardiomyocytes. NCX current (INCX), measured with whole cell patch clamp, was inhibited in response to cytosolic NADH loaded directly via pipette or increased by extracellular lactate perfusion, whereas an increase of mitochondrial NADH had no effect. Reactive oxygen species (ROS) accumulation was enhanced by increasing cytosolic NADH, and NADH-induced INCX inhibition was abolished by the H2O2 scavenger catalase. NADH-induced ROS accumulation was independent of mitochondrial respiration (rotenone-insensitive) but was inhibited by the flavoenzyme blocker diphenylene iodonium. NADPH oxidase was ruled out as the effector because INCX was insensitive to cytosolic NADPH, and NADH-induced ROS and INCX inhibition were not abrogated by the specific NADPH oxidase inhibitor gp91ds-tat. This study reveals a novel mechanism of NCX regulation by cytosolic NADH/NAD(+) redox potential through a ROS-generating NADH-driven flavoprotein oxidase. The mechanism is likely to play a key role in Ca(2+) homeostasis and the response to alterations in the cytosolic pyridine nucleotide redox state during ischemia-reperfusion or other cardiovascular diseases.

Delayed calcium dysregulation in neurons requires both the NMDA receptor and the reverse Na+/Ca2+ exchanger

Glutamate-induced delayed calcium dysregulation (DCD) is a causal factor leading to neuronal death. The mechanism of DCD is not clear but Ca2+ influx via N-methyl-d-aspartate receptors (NMDAR) and/or the reverse plasmalemmal Na+/Ca2+ exchanger (NCXrev) could be involved in DCD. However, the extent to which NMDAR and NCX(rev) contribute to glutamate-induced DCD is uncertain. Here, we show that both NMDAR and NCX(rev) are critical for DCD in neurons exposed to excitotoxic glutamate. In rat cultured hippocampal neurons, 25 μM glutamate produced DCD accompanied by sustained increase in cytosolic Na+ ([Na+]c) and plasma membrane depolarization. MK801 and memantine, noncompetitive NMDAR inhibitors, added shortly after glutamate, completely prevented DCD whereas AP-5, a competitive NMDAR inhibitor, failed to protect against DCD. None of the tested inhibitors lowered elevated [Na+]c or restored plasma membrane potential. In the experiments with NCX reversal by gramicidin, MK801 and memantine robustly inhibited NCXrev while AP-5 was much less efficacious. In electrophysiological patch-clamp experiments MK801 and memantine inhibited NCXrev-mediated ion currents whereas AP-5 failed. Thus, MK801 and memantine, in addition to NMDAR, inhibited NCXrev. Inhibition of NCXrev either with KB-R7943, or by collapsing Na+ gradient across the plasma membrane, or by inhibiting Na+/H+ exchanger with 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and thus preventing the increase in [Na+]c failed to preclude DCD. However, NCXrev inhibition combined with NMDAR blockade by AP-5 completely prevented DCD. Overall, our data suggest that both NMDAR and NCXrev are essential for DCD in glutamate-exposed neurons and inhibition of individual mechanism is not sufficient to prevent calcium dysregulation.

Alternative strategies in arrhythmia therapy: evaluation of Na/Ca exchange as an anti-arrhythmic target

The search for alternative anti-arrhythmic strategies is fueled by an unmet medical need as well as by the opportunities arising from identification of novel targets and novel drugs. Na/Ca exchange is a potential target involved in several types of arrhythmias, such as those related to ischemia-reperfusion, heart failure and also some forms of genetic arrhythmias. Inhibition of Na/Ca exchange is theoretically not only anti-arrhythmic but also increases cellular Ca(2+) content. This could be an advantage in conditions of low inotropy, such as in heart failure, but may also worsen conditions such as the recovery from ischemia or relaxation abnormalities. With the available drugs such as KB-R7943 and SEA-0400 these theories have now been tested in a number of cellular and in vivo models. Experience is overall rather positive and seems less hampered by the potential drawbacks than expected. This may be because the currently available drugs are not highly selective, with additional benefit derived from concurrent effects. While this precludes a definite answer regarding the benefit of a pure NCX inhibitor, they indicate that Na/Ca exchange inhibition as part of a multi-target strategy is an avenue to be considered. Such studies will need further 'bench' work and testing in relevant preclinical models, including chronic disease.

Preconditioning with Na+/H+ exchange inhibitor HOE642 reduces calcium overload and exhibits marked protection on immature rabbit hearts

Inhibition of Na/H exchanger isoform-1 (NHE1) has shown significant protection in adult myocardium during ischemia/reperfusion injury; however, the effect is unclear in immature myocardium. We evaluated the effects of HOE642 (a potent, highly selective NHE1 inhibitor) preconditioning on immature rabbit hearts. Twenty immature (2-3 weeks old) New Zealand white rabbits were randomly divided into the control group (n = 10) and the HOE642 preconditioning group (n = 10). The immature isolated hearts were subjected to 45 minutes of normothermic global ischemia plus 60 minutes of reperfusion after being established on the Langendorff apparatus. During reperfusion, the recovery rates of cardiac function (LVDP, +dp/dtmax, -dp/dtmax, and coronary flow) were about 90% in the HOE642 treated group and about 50% in the control group (p < 0.05). HOE642 preconditioning can significantly decrease the release of cardiac specific enzymes CK, CK-MB and LDH (p < 0.05) and the myocardial water content (p < 0.05). Meanwhile, HOE642 markedly attenuated intracellular calcium overload (265.8 +/- 41.1 vs. 500.7 +/- 60.8 mg/kg dry wt) (p < 0.01). The blinded ultrastructural assessment under transmission electron microscopy illustrated that preconditioning with HOE642 produced evident myocyte salvage. This study demonstrates that preconditioning with HOE642 provides a significant protection during ischemia/reperfusion injury in immature myocardium, mostly by reducing myocardial calcium overload.

Effect of partial blockade of the Na(+)/Ca(2+)-exchanger on Ca(2+) handling in isolated rat ventricular myocytes

SEA0400 is a selective inhibitor of the Na(+)/Ca(2+) exchanger having equal potencies to suppress both the forward and reverse mode operation of the Na(+)/Ca(2+) exchanger. Present experiments were designed to study the effect of partial blockade of Na(+)/Ca(2+) exchanger on Ca(2+) handling in isolated rat ventricular myocytes. Intracellular Ca(2+) transient and cell shortening were measured in ventricular myocytes loaded with Fura-2-AM fluorescent dye. Partial blockade of Na(+)/Ca(2+) exchanger was induced by superfusion of the cells with SEA0400 at a concentration of 0.3 microM. Amplitude of the intracellular Ca(2+) transient and cell shortening was significantly increased by SEA0400 in both field stimulated and voltage clamped myocytes, without significant elevation of diastolic Ca(2+) level and the decay time constant of the Ca(2+) transient. In patch clamped myocytes the SEA0400 induced increase in the Ca(2+) transient and cell shortening was accompanied by significant reduction of peak L-type Ca(2+) current. These effects can be explained by the autoregulative nature of cardiac Ca(2+) handling, as the reduced Ca(2+) efflux from the cell results in an increased Ca(2+) load to the sarcoplasmic reticulum leading to increased Ca(2+) release, which in turn may decrease the L-type Ca(2+) current by accelaration of Ca(2+) dependent inactivation of L-type Ca(2+) current. Our results suggest that complex changes in the Ca(2+) cycling can occur after selective pharmacological inhibition of the Na(+)/Ca(2+) exchanger.

Postischemic infusion of 17-beta-estradiol protects myocardial function and viability

BACKGROUND

Females demonstrate improved cardiac recovery after ischemia/reperfusion injury compared with males. Attenuation of myocardial dysfunction with preischemic estradiol suggests that estrogen may be an important mediator of this cardioprotection. However, it remains unclear whether post-injury estradiol may have clinical potential in the treatment of acute myocardial infarction. We hypothesize that postischemic administration of 17beta-estradiol will decrease myocardial ischemia/reperfusion injury and improve left ventricular cardiac function.

MATERIALS AND METHODS

Adult male Sprague Dawley rat hearts (n = 20) (Harlan, Indianapolis, IN) were isolated, perfused with Krebs-Henseleit solution via Langendorff model, and subjected to 15 min of equilibration, 25 min of warm ischemia, and 40 min reperfusion. Experimental hearts received postischemic 17beta-estradiol infusion, 1 nm (n = 4), 10 nm (n = 4), 25 nm (n = 4), or 50 nm (n = 4), throughout reperfusion. Control hearts (n = 4) were infused with perfusate vehicle.

RESULTS

Postischemic recovery of left ventricular developed pressure was significantly greater with 1 nm (51.6% +/- 7.4%) and 10 nm estradiol (47.7% +/- 8.6%) than with vehicle (37.8% +/- 9.7%) at end reperfusion. There was also greater recovery of the end diastolic pressure with 1 nm (47.8 +/- 4.0 mmHg) and 10 nm estradiol (54.0 +/- 4.0) compared with vehicle (75.3 +/- 7.5). Further, 1 nm and 10 nm estrogen preserved coronary flow after ischemia and decreased coronary effluent lactated dehydrogenase compared with controls. Estrogen at 25 nm and 50 nm did not provide additional benefit in terms of functional recovery. Estrogen at all concentrations increased extracellular signal-regulated protein kinase phosphorylation.

CONCLUSIONS

Postischemic infusion of 17beta-estradiol protects myocardial function and viability. The attractive potential for the clinical application of postischemic estrogen therapy warrants further study to elucidate the mechanistic pathways and differences between males and females.

Reduction by SEA0400 of myocardial ischemia-induced cytoplasmic and mitochondrial Ca2+ overload

The cardioprotective effects of SEA0400, a novel Na(+)-Ca(2+) exchanger inhibitor, were examined in isolated guinea pig myocardial tissue and ventricular myocytes. In a coronary-perfused right ventricular tissue preparation, SEA0400 had no cardiosuppressive effect during normoxia and experimental ischemia, but enhanced the recovery of contractile force during reperfusion. SEA0400 had no effect on tissue ATP content during normoxia, but attenuated its decrease during ischemia. Treatment of ventricular myocytes with an ischemia mimetic solution (high K(+), glucose free, pH 6.0, gassed with N(2)) resulted in the depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca(2+) concentration, which had a similar time course. SEA0400 significantly delayed these changes. These results suggest that SEA0400 maintains mitochondrial function and tissue ATP content during ischemia through the inhibition of cytoplasmic and mitochondrial Ca(2+) overload.